Plants and Soils

The development of soils is an often overlooked aspect of the Devonian Transformation. However, changes in weathering and pedogenesis (soil formation) during this time had profound effects on terrestrial, freshwater and marine environments. They may have even contributed to a global mass extinction and changed the climate.

Weathering is the process of breaking parent rock material into smaller fragments and ultimately into soil. Weathering can and does occur without plants, but it occurs more slowly and is dominated by physical processes (e.g., abrasion and freeze-thaw fracturing). Physical weathering generally results in coarser sediment size (i.e., the soil is sandy and rocky) in part because the finer sediments are lost to water and wind erosion.

Vascular plants can increase physical weathering by reducing erosion and thereby increasing the time during which the sediments can be physically altered in sutu. However, vascular plants also influence weathering via chemical processes. They do this by the direct and indirect production and delivery of organic acids. These acids are produced by mycorhizae (symbiotic root fungi), bacterial decomposition of plant matter and the abiotic oxidation of plant matter. Plants deliver these organic acids via leaching of surface litter and penetration of the sediments by roots. These processes, also known as chemical weathering, greatly increases the proportion of finer particles, especially clays.

Land surfaces prior to the Middle Silurian were probably either exposed bedrock or thin microbial protosoils. Soil depths increase in the Late Silurian and Early Devonian, but depths are still relatively shallow and poorly horizonated. The first histsol (a thin but organic soil) is reported from the Early Devonian Rhynie Chert, a locality famous for its early vascular plants.

A progressive increase in soil depth and geographic extent during the Devonian and Early Carboniferous is strongly associated with the development of plant rooting systems. Root traces become more frequent and extensive during the Middle Devonian, but most are still relatively shallow (<20 cm). The situation changes dramatically in the Late Devonian with the appearance of Archaeopteris. These trees had extensive root systems that reached depths in excess of 1 m. Moreover, perennial secondary (i.e., thickening) growth and repeated production of lateral rootlets resulted in the more extensive penetration of a given volume of soils. Other Late Devonian plants such as arborescent lycopsids (Cyclostigma and Lepidendropsis/Protostigmaria), Rhacophyton and early seed plants also created extensive, albeit shallower root systems.

Clay-rich vertisols have been reported from pre-Devonian localities, but they become more frequent during the Devonian (e.g., Red Hill) and very common in the Carboniferous. Several new soil types (alfisols, ultisols and spodsols) also make their first appearance either in the Late Devonian or Early Carboniferous. Moreover, the increase abundance of a variety of clay minerals (e.g., kaolinite, laterite and smectite) during this interval indicates enhanced chemical weathering and pedogenic processes. In summary, increases in chemical weathering, soil depth, clay content, soil profile structuring, and geographic extent are strongly associated with the spread of vascular land plants.

Several consequences of enhanced pedogenesis are worth noting. One, of course, is that deeper and more developed soils enhance plant growth (or production). Increased plant production, in turn, fosters greater production by terrestrial animals and results in increased organic matter exports to the streams and ultimately the oceans. Secondly, the increasingly deep and fine-textured soils of the Devonian augmented the stabilizing influences of vegetation in moderating hydrologic regimes by absorbing larger quantities of stormwaters; flooding became less destructive. Thirdly, the finer clays and muds from these developing soils helped stabilize stream channels.

However, one of the most dramatic consequences of these emerging soils may have been the drawdown of atmospheric carbon dioxide (CO2). Increased residence times for Late Devonian and Early Carboniferous soils meant that they were subject to greater silicate weathering (i.e., the conversions of calcium and magnesium silicates into carbonates). Eventually, dissolved calcium and magnesium carbonates would travel down the streams to the oceans where they would precipitate and be removed from the global carbon cycle. A new equilibrium between carbon burial (including both organic carbon and inorganic carbonates) and geologic carbon release (e.g., volcanic activity) established itself by the Early Permian, but the atmospheric concentrations of CO2 would be set at a much lower concentration. Estimating early Paleozoic concentrations is still an imprecise science but there is general agreement among three very different methods: carbon isotope analyses of marine carbonates (e.g., dolomites and limestones), carbon isotope analyses in paleosoils, and stomatal density in fossil plants. Estimates of Late Silurian atmospheric CO2 concentrations are 10-18 times higher than they are today (~270 ppm), while Late Devonian estimates are only 2-5 times higher.